A titanosilicate, also called titanium silicate or silicotitanate, is a
silicate mineral
Silicate minerals are rock-forming minerals made up of silicate groups. They are the largest and most important class of minerals and make up approximately 90 percent of Earth's crust.
In mineralogy, the crystalline forms of silica (silicon dio ...
where some portion of the
silicon
Silicon is a chemical element; it has symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic lustre, and is a tetravalent metalloid (sometimes considered a non-metal) and semiconductor. It is a membe ...
atoms have been replaced by
titanium(IV) atoms. The term most commonly refers to the
zeolitic polymorph, also called titanium
silicalite, which is a
heterogeneous catalyst
Heterogeneous catalysis is catalysis where the phase of catalysts differs from that of the reagents or products. The process contrasts with homogeneous catalysis where the reagents, products and catalyst exist in the same phase. Phase distingui ...
in
industrial chemistry
The chemical industry comprises the company, companies and other organizations that develop and produce industrial, specialty and other chemicals. Central to the modern world economy, the chemical industry converts raw materials (Petroleum, oil, ...
.
Applications
Titanosilicate catalysts are commonly used in
oxidation reaction
Redox ( , , reduction–oxidation or oxidation–reduction) is a type of chemical reaction in which the oxidation states of the reactants change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is ...
s that employ
hydrogen peroxide
Hydrogen peroxide is a chemical compound with the formula . In its pure form, it is a very pale blue liquid that is slightly more viscosity, viscous than Properties of water, water. It is used as an oxidizer, bleaching agent, and antiseptic, usua ...
as an
oxidant
An oxidizing agent (also known as an oxidant, oxidizer, electron recipient, or electron acceptor) is a substance in a redox chemical reaction that gains or "Electron acceptor, accepts"/"receives" an electron from a (called the , , or ''electr ...
. The production of
propylene oxide
Propylene oxide is an epoxide with the molecular formula C3H6O. This colourless volatile liquid with an odour similar to ether, is produced on a large scale industrially. Its major application is its use for the production of polyether polyols f ...
by
epoxidation
In organic chemistry, an epoxide is a cyclic ether, where the ether forms a three-atom ring: two atoms of carbon and one atom of oxygen. This triangular structure has substantial ring strain, making epoxides highly reactive, more so than other ...
of
propylene
Propylene, also known as propene, is an unsaturated organic compound with the chemical formula . It has one double bond, and is the second simplest member of the alkene class of hydrocarbons. It is a colorless gas with a faint petroleum-like o ...
is a major industrial application. Epoxidation by is considered a
green
Green is the color between cyan and yellow on the visible spectrum. It is evoked by light which has a dominant wavelength of roughly 495570 nm. In subtractive color systems, used in painting and color printing, it is created by a com ...
er alternative to epoxidation via
propylene chlorohydrin, but this reaction of and propylene competes with many
side reaction
A side reaction is a chemical reaction that occurs at the same time as the actual main reaction, but to a lesser extent. It leads to the formation of by-product, so that the Yield (chemistry), yield of main product is reduced:
: + B ->[] P1
: + C ...
s. The formation of titanium coordination complexes in the zeolite structure improves selectivity for the epoxide.
Synthesis
Titanosilicates can be prepared via
crystallisation of
silica
Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , commonly found in nature as quartz. In many parts of the world, silica is the major constituent of sand. Silica is one of the most complex and abundant f ...
and
titanium dioxide
Titanium dioxide, also known as titanium(IV) oxide or titania , is the inorganic compound derived from titanium with the chemical formula . When used as a pigment, it is called titanium white, Pigment White 6 (PW6), or Colour Index Internationa ...
with
quaternary ammonium cation
In organic chemistry, quaternary ammonium cations, also known as quats, are positively-charged polyatomic ions of the structure , where R is an alkyl group, an aryl group or organyl group. Unlike the ammonium ion () and the primary, secondary, ...
s as a structure-directing agent. The silica and titanium dioxide are provided by
hydrolysis
Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution reaction, substitution, elimination reaction, elimination, and solvation reactions in which water ...
of
tetraethyl orthosilicate
Tetraethyl orthosilicate, formally named tetraethoxysilane (TEOS), ethyl silicate is the organic chemical compound with the formula Si(OC2H5)4. TEOS is a colorless liquid. It degrades in water. TEOS is the of orthosilicic acid, Si(OH)4. It ...
and
tetrabutyl orthotitanate.
Structure
TS-1, the major industrial titanosilicate, has the MFI crystal structure seen in
ZSM-5
ZSM-5, Zeolite Socony Mobil–5 (framework type MFI from ZSM-5 (five)), is an aluminosilicate zeolite belonging to the pentasil family of zeolites. Its chemical formula is NanAlnSi96–nO192·16H2O (0tetrahedral
In geometry, a tetrahedron (: tetrahedra or tetrahedrons), also known as a triangular pyramid, is a polyhedron composed of four triangular Face (geometry), faces, six straight Edge (geometry), edges, and four vertex (geometry), vertices. The tet ...
ly coordinated, unlike the
octahedral
In geometry, an octahedron (: octahedra or octahedrons) is any polyhedron with eight faces. One special case is the regular octahedron, a Platonic solid composed of eight equilateral triangles, four of which meet at each vertex. Many types of i ...
coordination found in the main polymorphs of
titanium(IV) oxide. Because of the relatively small amount of titanium in the relatively large structure, it is difficult to determine the exact locations of active sites; this is a major obstacle in understanding the
reaction mechanism
In chemistry, a reaction mechanism is the step by step sequence of elementary reactions by which overall chemical reaction occurs.
A chemical mechanism is a theoretical conjecture that tries to describe in detail what takes place at each stage ...
of titanosilicate catalysis.
Other titanosilicates have been produced in other zeolite structures, as well as related mesoporous materials such as Ti-
MCM-41.
History
The first synthetic titanosilicate zeolite, TS-1, was patented by Taramasso et al. in 1983. TS-1 was first used industrially by
EniChem in 1986, for the
hydroxylation
In chemistry, hydroxylation refers to the installation of a hydroxyl group () into an organic compound. Hydroxylations generate alcohols and phenols, which are very common functional groups. Hydroxylation confers some degree of water-solubility ...
of
phenol
Phenol (also known as carbolic acid, phenolic acid, or benzenol) is an aromatic organic compound with the molecular formula . It is a white crystalline solid that is volatile and can catch fire.
The molecule consists of a phenyl group () ...
. They would later expand its usage into the production of
cyclohexanone oxime. TS-1 as a catalyst for epoxidation of propylene was first deployed at industrial scale by
Dow Chemical
The Dow Chemical Company is an American multinational corporation headquartered in Midland, Michigan, United States. The company was among the three largest chemical producers in the world in 2021. It is the operating subsidiary of Dow Inc., ...
and
BASF
BASF SE (), an initialism of its original name , is a European Multinational corporation, multinational company and the List of largest chemical producers, largest chemical producer in the world. Its headquarters are located in Ludwigshafen, Ge ...
in 2008.
References
{{reflist
Silicate minerals